Australian scientists are claiming a first in super-cool roof technology, with a surface that stays cooler than the ambient air temperature even under full summer sun.
They claim their discovery could reduce the urban heat island effect in cities and cut peak power demand from air-conditioning.
The special material is a "coated polymer stack" – a combination of specially chosen plastics on a silver layer.
"We demonstrate for the first time how to make a roof colder than the air temperature around it, even under the most intense summer conditions," said Emeritus Professor Geoff Smith from the University of Technology Sydney (UTS), co-author of the study.
Roofs heat up by absorbing sunlight, so darker roofs can get very hot. Even white roofs still absorb enough sunlight to warm up by as much as 12°C.
An infrared image showing the temperature difference between the new surface (centre) and an existing cool roof used in testing (UTS)
This new surface stayed 11°C or more colder than a state-of-the-art white roof nearby because it absorbs only 3% of sunlight while radiating heat at infrared wavelengths that are not absorbed by the atmosphere.
The plastic materials used for the demonstration were readily available and could potentially be used for roofing.
Smith said that cooling a roof below ambient air temperature had until now been "an elusive target", and that the implications could be profound, since much of the world’s population inhabits warm climates.
"Cool roofing reduces the severity of the urban heat island problem in towns and cities and helps eliminate peak power demand problems from the operation of many air conditioners," he said.
The test roof was placed on top of the UTS Faculty of Science building in full exposure to the sun.
The researchers also assessed the impact of grime build-up on the test roof because, being cooler, it would cause dew formation and collect dust. They said that although it was above a busy city road, its "excellent thermal performance" was maintained.
The paper, "A Sub-ambient Open Roof Surface Under the Mid-summer Sun", co-authored by Dr Angus Gentle, appears in the latest edition of Advanced Science.
Main image: Dr Angus Gentle holding a piece of the special material over an existing cool roof used in testing